mmc: core: don't try to switch block size for dual rate mode

Per spec, block size should always be 512 bytes for dual rate mode,
so any attempts to switch the block size under dual rate mode should
be neglected.

Signed-off-by: Ziyuan Xu <xzy.xu@rock-chips.com>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>